Compound change gear transmissions are typically associated with heavy duty vehicles such as large trucks, tractor/semi-trailers, and the like. Compound transmissions comprise main and auxiliary transmission sections connected in series, and provide a total number of available transmission ratios equal to the product of the main and auxiliary section ratios. By way of example, a compound change gear transmission comprising a four (4) speed main section connected in series with a four (4) speed auxiliary section will provide sixteen (4.times.4=16) available ratios.
Power is transmitted from the engine, through the master clutch and into the transmission via the input shaft. In "Fuller" transmissions, a gear is splined to the input shaft (namely the headset gear) which is permanently enmeshed with two countershaft gears situated 180.degree. apart on the periphery of the headset gear. Torque is transmitted to the countershaft gears and subsequently through the countershafts. The countershafts generally include a number of gears which mate with mainshaft gears which are placed along the same axis of the input shaft, free to float on a floating mainshaft. Clutches are provided between the mainshaft and the mainshaft gears to provide progressive ratios. By moving a clutch from its neutral position to an engaged position, torque is transmitted from the countershafts, into the mainshaft gear, through the clutch and into the mainshaft. This method splits the input torque equally between the countershafts and brings the geared torque back into the mainshaft gear, again split equally.
For the torque to be split equally and effectively, it is important that the mainshaft, mainshaft gears, and clutches are able to float to assume centered positions. It is not necessary to firmly fix the mainshaft gears and mainshaft, as is common practice with single layshaft transmissions, since the separating and tangential forces generated at the gear teeth are equal and opposite and therefore cancel each other out. In fact, fixing the mainshaft/input shaft gears can be detrimental and produce a torque imbalance because it is impossible to manufacture the geartrain perfectly, i.e., to absolute sizes without tolerance. The manufactured tolerances can result in the gear teeth of the mainshaft and input shaft gears being more heavily loaded on one side than the other, and consequently, the gears on one countershaft are loaded more than on the other. Furthermore, this can give rise to gear tipping problems and, in extreme cases, gear hopout during normal driving conditions.
In a single layshaft, medium-duty transmission, torque is supplied into the transmission via the input shaft through the headset gear, and is passed from the headset gear to a mating layshaft gear, and into the single layshaft. In this case, the mainshaft is simply supported with bearings with very little radial clearance. The mainshaft gears are held concentric with the mainshaft on needle roller bearings. This is necessary due to the high tangential and separating forces set up between the two mating gears which must be reacted through rolling elements into the transmission housing.
Compound transmissions are sometimes used for "power take off" (PTO) operation in which torque is transmitted from one of the countershafts to an ancillary unit, such as a pump or flange device for operating a truck bed lifter, etc. The headset gear of a conventional Fuller twin countershaft transmission is splined to the input shaft with a small diametral float, and when a PTO is fitted to the drive from the front countershaft, the transmission is utilized in a single layshaft manner. The headset gear or mainshaft gears are not coupled to the mainshaft, hence no torque split. The headset gear drives torque through one countershaft only. Since the headset gear is not coupled with the clutch, there cannot be any gear hopout. The small diametral clearance ensures that the headset gear runs concentric with the input shaft and the large drive splines are strong enough to sustain the load cycles during PTO operation.
In certain designs, the splitter is configured such that the headset gear, which is usually mounted on the input shaft via a spline, can be free to rotate and float, and performs the low split function (on an overdrive transmission) as well as the fourth gear function. This gear is mounted on a spindle which is screwed into the mainshaft. The headset gear bore includes a clearance fit to the spindle so that it may float under normal driving operation to ensure a balanced torque split. It is also supported axially by cylindrical thrust roller bearings which compensate for the axial thrust forces apparent during normal operation due to the helical gearing, which is not balanced. These forces are accompanied by a differential rotation between the headset gear and spindle when the low split gear is selected (i.e., when it is a driving gear), hence the need for thrust bearings. A splitter gear is then placed on the input shaft, forward of the headset/fourth gear with a plain clearance bore to the input shaft which provides a high split function. These two gears are then selectable using a splitter clutch splined to the input shaft and free to slide along the spline to supply the clutching function.
This design is operational for normal driving conditions, however, when these two gears are used for PTO operation, there is a tendency for the clutch to hopout of engagement. In essence, the reason for this hopout condition, both at high and low split, is due to the fact that the transmission is being used as a single layshaft transmission without bearings under the driving gear.
The hopout can be attributed to inadequate parallelism and concentricity between the selected splitter or headset gear, splitter clutch and shaft on which the gear is situated. The gear becomes displaced radially taking up the clearance between the gear bore and shaft at the high split position, and similarly at the low split position but with the added clearance that exists due to the floating nature of the mainshaft and spindle. This results in a tipping affect.
The need exists for providing such torque splitting options, while also providing the availability of PTO operation corresponding with both the splitter gear and the headset/fourth gear. It is necessary to provide gear float for balanced torque splitting, however, unfortunately, this float allows greater gear tipping, particularly during PTO operation, which increases the likelihood of gear hopout at the splitter clutch.
It is desirable to provide a compound transmission design which provides substantially balanced torque splitting, with the availability of PTO operation corresponding with both the splitter gear and headset/fourth gear without increasing the risk of gear hopout during PTO operation.